A prior art infrared motion detector device essentially comprises an arc lens assembly, and a pyro-sensor with electrical circuitry. The arc lens is made up of high-density polyethylene (HDPE) polymeric material. Optical focal points can be designed onto the lenses in Fresnel or dotted configuration. A stacked-up multifaceted arc lens assembly made from Fresnel configuration comprises a plurality of optical segments with individual focus arranged in layers, making up a focusing view. A stacked-up multifaceted arc lens assembly made from dotted configuration comprises a plurality of optical focuses provided in optical segments in layers, making up a focusing view. A focusing view is determined by the angle of detection, up to a maximum of 360 degrees, as well as the detection range. This can be considered as the detection coverage.
During installation to cover an area, an installer will consider how far the motion detector device should cover and how wide the motion detector device should receive infrared rays. A focusing zone is therefore covered by the angle of detection and the range/distance of detection in the focusing view.
There are associated problems with prior art methods of adjusting the detection range. One method is to adjust the electronic sensitivity, with the assistance of a variable resistance knob. Since the turning of the knob is not calibrated with the distance, field adjustment requires a lot of trials and errors. Another method is to adjust the detection device by rotating horizontally and tilting vertically. As seen in FIG. 1, horizontal rotation is achieved with the assistance of a rotation bracket (33). As seen in FIG. 2, vertical tilting is achieved with the assistance of a swivel joint (35). Since the rotating or tilting is not calibrated with the distance, field adjustment also requires a lot of trials and errors. 
The prior art lens of a prior art motion detector device comprises optical layers where top layers cover a longer distance. To render a short-range detection, the top layers are usually masked or covered. As seen in FIG. 3, the lenses (52) are semi-spherically shaped and a lens masking means (53) hinged at both sides of the lenses (52) is used to cover selectively a layer or more of the lenses (52). The use of lens masking means (53) can take the form of masking tape or snap-on plastic sheet. This masking usually leads to a discounted visual appearance of overall product aesthetics after installation. This is especially true if the motion detector device is integrated with a decorative lighting fixture, in which the recommended mounting height is about 1.8 m typically next to an entrance of a premise or the doorway of a building. 
There are associated problems with prior art methods of adjusting angle of detection coverage. The angle of detection is determined by the design (known as field of view) of pyro-sensor used in the motion detector device as well as the focusing view of the lenses. In a situation where a motion detector device is aimed to cover a narrow territory such as a passageway next to a public service road, any motion within the focusing zone covered i.e. the passageway would emit infrared rays, which should be picked up within the focusing view of the lenses and subsequently the field of view of pyro-sensor. Any infrared rays in the public service road should not be detected, otherwise a “false alarm” would be triggered. One usual method is to mask the unwanted side segments of the optical lenses. The focusing view of the lenses is then curtailed, so that only infrared rays from the focusing zone enter the narrow focusing view of the lenses. Alternatively, a lens masking means (53) is employed to selectively cover a segment of the semi-spherically shaped lenses as seen in FIG. 3. 
The main disadvantage of prior art motion detector devices is that the lens assembly is substantially permanent and only one focusing view is therefore designed for use. This feature does not facilitate on-site selection flexibility. In practice, depending on the actual physical orientation of the premise to be covered, a prior art with a fixed focusing view may not be optimally used for installation. Hence, where there are different prior art motion detector devices with different fixed focusing views, different spare parts need to be manufactured and the cost invariably goes up.